Systems having digestion units with capability for loading cellulosic biomass under high pressure and associated methods for cellulosic biomass processing

Abstract

When processing cellulosic biomass, it may be desirable for a digestion unit to operate without being fully depressurized for process efficiency purposes. A biomass conversion system can comprise a first digestion unit and a second digestion unit that are operatively connected to one another; a valve separating the first digestion unit from the second digestion unit; a fluid circulation loop establishing fluid communication between an outlet of the first digestion unit and an inlet of the second digestion unit; and a bypass line establishing fluid communication between an outlet of the second digestion unit and the fluid circulation loop.

What is claimed is:  
     
       1. A method comprising:
 (a) providing biomass conversion system comprising:
 i. a first digestion unit and a second digestion unit that are operatively connected to one another; 
 
 (b) at least partially digesting a first portion of cellulosic biomass in the second digestion unit, thereby forming a hydrolysate comprising soluble carbohydrates within a liquor phase; 
 (c) providing pressure isolation between the first digestion unit and the second digestion unit; 
 (d) after pressure isolation, at least partially depressurizing the first digestion unit and loading the first digestion unit with a second portion of cellulosic biomass while digestion continues in the second digestion unit; 
 (e) re-pressurizing the first digestion unit to a pressure less than or equal to a pressure in the second digestion unit; 
 (f) after said re-pressurization, equalizing the pressure between the first digestion unit and the second digestion unit; and 
 (g) after said pressure equalization, transferring at least a portion of the cellulosic biomass from the first digestion unit to the second digestion unit. 
 
     
     
       2. The method of  claim 1 , further comprising:
 continuing the digestion of the cellulosic biomass in the second digestion unit at a pressure of at least about 30 bar. 
 
     
     
       3. The method of  claim 1 , wherein the re-pressurizing of the first digestion unit is achieved at least by transferring a liquor phase from a surge vessel to the first digestion unit. 
     
     
       4. The method of  claim 1 , further comprising:
 circulating the liquor phase through a fluid circulation loop between an outlet of the first digestion unit and an inlet of the second digestion unit. 
 
     
     
       5. The method of  claim 4 , wherein the fluid circulation loop and the second digestion unit are configured to establish countercurrent flow in the second digestion unit. 
     
     
       6. The method of  claim 1 , further comprising:
 converting the hydrolysate into a biofuel. 
 
     
     
       7. The method of  claim 1 , wherein at least about 60% of the cellulosic biomass, on a dry basis, is digested to produce hydrolysate. 
     
     
       8. The method of  claim 1 , wherein at least about 90% of the cellulosic biomass, on a dry basis, is digested to produce hydrolysate. 
     
     
       9. The method of  claim 1 , wherein the cellulosic biomass comprises wood. 
     
     
       10. The method of  claim 1 , wherein the second digestion unit is maintained at a pressure of at least about 30 bar. 
     
     
       11. The method of  claim 1 , wherein the second digestion unit is maintained at a pressure of at least about 50 bar. 
     
     
       12. The method of  claim 1 , wherein the second digestion unit is maintained at a pressure of at least about 70 bar. 
     
     
       13. The method of  claim 1 , wherein re-pressurizing the first digestion unit takes place, at least in part, by introducing at least a portion of the liquor phase from the second digestion unit to the first digestion unit. 
     
     
       14. The method of  claim 13 , wherein re-pressurizing the first digestion unit takes place, at least in part, by introducing a gas into the first digestion unit. 
     
     
       15. A method comprising:
 at least partially digesting a cellulosic biomass contained in a first digestion unit and a second digestion unit to produce a hydrolysate comprising soluble carbohydrates in a liquor phase, the first digestion unit and the second digestion unit being operatively connected to one another; 
 circulating the liquor phase from the first digestion unit to the second digestion unit through a fluid circulation loop establishing fluid communication between an outlet of the first digestion unit and an inlet of the second digestion unit; 
 isolating the first digestion unit from the second digestion unit, the liquor phase continues to flow through the second digestion unit to the fluid circulation loop via a bypass line establishing fluid communication between an outlet of the second digestion unit and the fluid circulation loop; 
 while hydrolysis continues in the second digestion unit, adding a cellulosic biomass to the first digestion unit and pressurizing the first digestion unit to a pressure that is less than or equal to a pressure in the second digestion unit; 
 equalizing the pressure between the first digestion unit and the second digestion unit; and 
 transferring at least a portion of the cellulosic biomass from the first digestion unit to the second digestion unit. 
 
     
     
       16. The method of  claim 15 , further comprising:
 continuing the digestion of the cellulosic biomass in the second digestion unit at a pressure of at least about 30 bar. 
 
     
     
       17. The method of  claim 15 , further comprising:
 converting the hydrolysate into a biofuel. 
 
     
     
       18. The method of  claim 15 , wherein the biomass conversion system further comprises at least one surge vessel in fluid communication with the outlet of the first digestion unit and located within the fluid circulation loop. 
     
     
       19. The method of  claim 15 , wherein the fluid circulation loop and the second digestion unit are configured to establish countercurrent flow in the second digestion unit. 
     
     
       20. The method of  claim 15 , wherein at least about 60% of the cellulosic biomass, on a dry basis, is digested to produce hydrolysate. 
     
     
       21. The method of  claim 15 , wherein at least about 90% of the cellulosic biomass, on a dry basis, is digested to produce hydrolysate. 
     
     
       22. The method of  claim 15 , wherein the cellulosic biomass comprises wood. 
     
     
       23. The method of  claim 15 , wherein the second digestion unit is maintained at a pressure of at least about 30 bar. 
     
     
       24. The method of  claim 15 , wherein the second digestion unit is maintained at a pressure of at least about 50 bar. 
     
     
       25. The method of  claim 15 , wherein the second digestion unit is maintained at a pressure of at least about 70 bar. 
     
     
       26. The method of  claim 15 , wherein pressurizing the first digestion unit takes place, at least in part, by introducing at least a portion of the liquor phase to the first digestion unit. 
     
     
       27. The method of  claim 26 , wherein pressurizing the first digestion unit takes place, at least in part, by introducing a gas into the first digestion unit.